The use of rotating polygon scanning mirrors in laser printers to provide a beam sweep or scan of the image of a modulated light source across a photorestive medium such as a rotating drum is well known. More recently, there have been efforts to use a much less expensive flat mirror with a single reflective surface such as a mirror oscillating in resonance to provide the scanning beam. These scanning mirrors provide excellent performance at a very advantageous cost. Unfortunately, the resonant frequency of the mirror as it pivots about its torsional hinges is highly susceptible to stresses that cause tension or compression of the hinges. Robust mounting brackets are typically used to mount the torsional hinge mirrors to a using device. However, distortion of the bracket itself due to mounting stresses can produce sufficient stress in the mirror hinges that will cause the resonant frequency of the scanning mirror to change beyond acceptable limits or even destroy the mirror.
Therefore, a mounting bracket that limits or substantially eliminates stresses transmitted to the mirror hinges is needed.
Texas Instruments presently manufactures mirror MEMS devices fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100 to 115 microns using semiconductor manufacturing processes. The reflective surface of the mirror may have any suitable perimeter shape such as oval, elongated elliptical, rectangular, square or other. Single axis mirrors include the reflective surface and a pair of torsional hinges, which extend to a support frame or alternately the hinges may extend from the mirror portion to a pair of hinge anchors.
U.S. patent application Ser. No. 10/384,861 describes various techniques for creating the pivotal resonance of the mirror device about the torsional hinges. Thus, by designing the mirror hinges to resonate at a selected frequency, a scanning engine can be produced that provide a scanning beam sweep with only a small amount of energy required to maintain resonance. However, as will be appreciated, the resonant frequency of a pivotally oscillating device about torsional hinges will vary as a function of the stress loading along the axis of the hinges. For example, when the mirror bracket is bolted in place, uneven mounting surfaces can cause the bracket to deform, thereby stressing the mirror hinges and causing a shift in the resonant frequency. Such stressing of the mirror hinges may cause a drift in the resonant frequency of the pivotal oscillations beyond tolerable limits or may actually destroy the mirror.
Since applications that use a scanning light beam, such as a laser printer, and an imaging projector require a stable precise drive to provide a single frequency scan velocity, the changes in the resonant frequency and scan velocity of a pivotally oscillating device due to temperature variations can restrict or even preclude the use of the device in laser printers.